Selective amplifier system



May 2, 1950 E. TOTH SELECTIVE AMPLIFIER SYSTEM Filed Aug. 3, 1945 3Sheets-Sheet 1 EMERICK TOTH May -2,' 1950 E. TOTH SELECTIVE AMPLIFIERSYSTEM 3 Sheets- Sheet 2 Filed Aug. 3, 1945 EMERICK TOTH May 2, 1950TQTH 2,505,813

SELECTIVE AMPLIFIER SYSTEM Filed Aug. 3, 1945 3 Sheets-Sheet 3 gi-vuq/wmEMERICK TOTH kw W Patented May 2, 1950 UNI-TE" STATES FATENT OFFICE 4Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) This-invention relatesto high frequency multistageselectiveamplifiersystems in which negative electromagnetic feedback isemployed to stabilize operation and thereby prevent regeneration- Anobject of this 'invention isto provide a high frequencybandselectiveamplifier and chassis layout' arrangedto preventregeneration producedas" a result'of. coupling between stages'or nonuniform groun'd potential,- whereby full use ofavailablechassis spaceisfattainable.

Another object of this invention-is to provide amulti-stage highfrequency amplifier system arranged so as I to-provide degenerativefeedback in combination with-means-for-controlling such feedback.

A--'further object of this invention is to provide a highfrequencyselective amplifier arranged so as to provide degenerativefeedbackproduced as aresult ofaelectromagnet c coupling between adjacentstages-together with means for controlling the amount of feedbac z, andfor minimizing feedback' between certaincomponent parts operating=at-anet phase difference which is somethingother.thanlSOdegrees.

Another object of-this invention is to reduce the length-of the chassisof a high frequency amplifier *so that it will be asmall percentage ofthewavelength employed, thus minimizing the radio frequency potentialdiiference which normally existsbetween opposite ends of the amplifier.

Other objects and. features of this invention will become apparent upona careful consideration of 'the. following detailed description when.

taken together-with the accompanying drawings.

Fig. l is acircuit diagram-of an intermediate frequency amplifierconnected'according to the principles of the: present invention;

Fig. 2 is-an elevationat view of the physical layout of some of -themajor-parts of the circuits f Fig. 1; and

Fig. 31 lea-perspective View showing in part Fig.2.

In general, the invention provides-a high frequency amplifier layout-anddesign in which negative- -electromagnetic feedback-is produced andcontrolledsto facilitate v the construction ofhigh frequency amplifiersystems. A sutable mounting-chassisis-provided on whichthe successivestages of amulti-stage amplifier may be arranged sov that the input andoutput satges lie in physicaLproximity. Interstageconnect'ons are madesothattan electrical phase difference of 180 existsbetweenthe inputandoutput stages. The external lead connections associatedwith these stagesare looped to accentuate the magnetic-flux produced thereby, andsuitable shielding meansare provided to control the amount and selectthephase of electromagneticfeedbackand thus preserve the gain of theamplifier.

With particular reference to'Fig. 1', the'circuit' diagram of a typicalmulti-stage intermediate frequency amplifier'system withinterstagetransformer coupling is d sclosed, comprising vacuum tubeslll; H, l2;it, M, 15, and-coupling transformers I8; 23; 3!), 36, 40; 41.

intermediate frequency amplifier design and are typifiedatstage I!) bythe cathode biasing combination 2B, 2!, the screen by-pass capacitance22 and decoupling resistance 22a,- the plate supply by-pass 24 anddecoupling resistance 24'a, andthe automatic volume control by-pass l9.and decoupling resistance l9a. Interstage transformers 23; 30, 36, 40,41are enclosed within shielding cans as'represented by the associateddotted lines and'are of thedoube tuned-variety to provide band-passaction. The input transformer l8; however, is single tuned, andlinkcoupled bymeans of winding: I 6- to an external signalsource (notshown) whose output is tobe amplifiedv A11 ground connections are madeto a pair of silver-plated copper groundingstrips which are placedinparallel relation on the underneath side of the main chassis member. and54 in combination with the main chassis 35 are shown in Figs. 2 and 3.The strips 53 and 54 are described in, copending application S. N.587,200, filed April 7, 1945, by Thomas M. Davis, entitled Electricalconnecting. fixture, and issued May 3, 1949, as U. S. PatentNo.2,468,737. As shown in Fig. 3, these strips comprise a body portion inwhich the tube socket and interstage transformerholes are cut, afirstintegral fiat shielding strip 8| extending at right angles to the bodyportion 8!], a second integral flat shielding, strip 82 alsoextending'at'right angles to. the body portion 8% and an integralinwardly extendingmember 83. Also as'shown in Fig. 3- the strips aremounted in parallelrelationship underneath a main chassis member 35andare electricallyjoined at one end. by a fiat metallic member. 551

To better illustrate theproduction of degenerativefeedback as a resultof couplingbetween input and output stages it is necessary to trace thephase shift'occurring through the amplifier. It is well knownthat thevoltage produced-across the secondary of an ordinary transformer bearsConnections to each stage are made according to'conventional- Thesestrips 53 a phase relationship of either degrees or 180 degrees withrespect to the voltage impressed across the primary depending upon thephasing of the leads to the windings. In a tuned transformer, however,the charging of the resonating capacitance placed across the secondaryof the transformer winding produces an additional phase displacement of90 degrees resulting in an overall phase displacement of the secondaryvoltage with respect to the primary voltage of either 90 degrees or 2'70degrees depending upon the connections to the transformer. In thisparticular illustration, all transformers have been connected in thecircuit in a manner that will produce a net displacement of 90 degreeswith the exception of transformer 41 which, for reasons shown later, isconnected to produce a net displacement of 2'70 degrees. The normalaction of a plate loaded vacuum tube results in the production, at theplate of that tube, of a voltage which is usually amplified and invertedwith respect to the signal applied to the grid. Thus, although obviouslyincorrect, it is commonly stated that, when operating upon a sine wavesignal, the tube produces an output voltage which is displaced from thegrid voltage by 180 degrees. This displacement, as is the case of allphase displacements hereinafter mentioned, is considered as a laggingphase displacement.

In determining the phase shift occurring between input and outputcircuits according to the principles outlined above, an initial or zerophase displacement condition may be assumed to exist at the inputwinding [6 shown in Fig. 1. The voltage applied to the grid of tube IDwill then be displaced by 90 degrees due to the action of transformerl8. An additional displacement of 180 degrees occurs in tube i0 causingthe voltage applied across the primary of transformer 23 to be displacedfrom that of the input circuit 16 by a total of 270 degrees. Thisaccumulation of phase shift may be followed through the amplifier whereit is evident that the voltage at the primary of transformer 41 isdisplaced from the voltage applied to the input winding l6 by a total of1350 degrees which may be considered equivalent to 2'70 degrees. Sincethe secondary voltage of transformer I8 is displaced 90 degrees withrespect to the input voltage, the net result is a phase displacement of180 degrees between the secondary of transformer l8 and the primary oftransformer 41.

As mentioned above, the connections to transformer 41 are such that the180 degree phase displacement between primary and secondary voltagesoccurs in addition to the 90 degree displacement. Thus a totaldisplacement of 1620 degrees or the equivalent of 180 degreesdisplacement exists between the voltage at the winding [6 and thevoltage across the detector l5.

To aid in the interpretation of this discussion, relative indications,arrowheads, are shown in Fig. 1 in proximity to the circuits to whichthey apply, oriented in a direction which is representative of therelative phase displacements existing in the circuit.

According to the invention the successive components as shown inparticular in Fig. 2, are positioned in a hairpin fashion, i. e. thestages of the amplifier are placed up one leg of the hairpin and downthe other on the parallel ground fixtures 53 and 54 so that the initialstages of the amplifier are opposite the final stages wherebyelectromagnetic feedback is produced,

Negative electromagnetic feedback between the input and output stagesmay be accentuated by proper placement of parts in parallel relationshipand by looping the associated lead Wires so as to form single turninductances. As indicated in Fig. 2, which is an under-side view of anactual layout of the amplifier, the plate and grid decouplingcapacitances, particularly the capacitances l 9, 24, 25, 46 and 50 inthe initial and final stages of the amplifier are mounted below thelevel of the tube sockets and transformer leads. As typified by theconnections to capacitance I9 and input transformer l 8, single turninductances are formed by each of these capacitances and the leads tothem in the following manner: One lead 90, for example, of thecapacitance I9 is grounded by its connection to the inwardly extendingfiat member 83 of the ground strip 53, the other lead 9! is connected tothe automatic volume control terminal of transformer l8. The grid lead92 from transformer I8 to the grid pin on the socket of tube I0 is inturn a loop connection. Magnetic flux linkage between these single turninductances as well as a certain amount of magnetic flux linkage betweenthe transformers l3 and 41 due to leakage through the transformer shieldcans produces the negative feedback by reason of their relative phaserelations. Although it is not absolutely necessary to the properoperation of the circuit, the plate and automatic volume controldecoupling capacitances are similarly looped for all stages for the sakeof uniformity.

As was previously shown, degenerative coupling exists between theprimary circuit of transformer is and the secondary circuit of 41 aswell as between the secondary circuit of i8 and the primary circuit of41. It was found that when the mounting strips 53 and 54 were positionedin very close proximity for optimum utilization of chassis space,excessive feedback resulted with an attendant distortion of thesymmetrical frequency selectivity characteristic. Therefore to controlthe negative feedback a partial shield H, which comprises a flatmetallic sheet, was fixed, screwed for example, over the primary leadsof the input transformer l8 as shown in Fig. 2 reducing the amount offeedback between the input loop l6 and the secondary circuit oftransformer 41.

In some instances it is desirable to omit the input transformer i8entirely, coupling directly to the grid of tube In. In this event thecoupling between the circuit of the detector [5 and the primary oftransformer l8 does not exist. Also the strong degree feedback componentbetween the circuit of detector l5 and the input circuit of tube ill isnot neutralized by a 90 degree component of opposite relationshipbetween the primary of transformer 41 and the primary of transformer l8.Although this 90 degree component cannot directly produce regenerativeaction at the mid-frequency of the selectivity curve, it is undesirablebecause it may produce some distortion of the shape of the frequencyselectivity characteristic or other ill-effects. It was found, however,that the effect of this field can be minimized by the insertion of ashie1d between input and output stages, tubes I0 and [-5 respectively.In the present illustration a simple fiat metallic member 5| screwed tothe main chassis member 35 and extending downward at right anglesthereto, for substantially the depth of the main chassis 35 and adjacentto the output stage [5 was found to be sufficient to suppress the 90degree components of the feedback and permit the 180 degree componentsbetween grid of tube l and plate of tube Hi to influence the operationof the circuit.

Furthermore it should be noted that the phase shifts as mentioned hereare correct for the midfrequency of the selectivity curve of thetunedcircuits at which point the tuned circuits act as pure resistancesproducing zero phase shifts. Either above or below the mid-frequency,the resonant circuits appear reactive in nature and therefore produceadditional phase shifts, Difiiculty in the phasing of the feedbackcomponents because of this additional shift is avoided primarily by theselectivity characteristics of the tuned circuits. At extremities on theselectivity curve, where added phase shifts would produce in-phasefeedback, the response of the amplifier is so low as to preventoscillation.

Fig. 3 is a perspective view of the underneath side of the amplifierchassis showing more clearly the location of the major parts and theirphys ical relationship, the partial shields 11, 5|, and the highconductivity grounding strips 53, 54, 55.

Where it is desired to sacrifice some overall gain for simplermechanical structure, the high conductivity grounding strips 53, 54, 55may be omitted, ground connections being made directly to the mainchassis member 35.

From the foregoing discussion it is apparent that considerablemodification of the basic invention is possible without exceeding thescope of the invention, therefore, the invention is not to be limitedexcept by the spirit of the prior art or the scope of the appendedclaims.

The invention described herein may be manufactured and used by or forthe Government 01' the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In combination, a chassis member, a multistage high frequencyamplifier system so positioned on said chassis that the input and outputcircuits of said amplifier lie in physical proximity one to the other,transformer type interstage connections between succeeding stages ofsaid amplifier, said interstage connections being poled to provide phaseopposed electromagnetic fields associated with said input and outputcircuits, selected lead wires in said input and output circuits beingdisposed beneath the chassis and looped in the form of single turninductances to provide degenerative coupling between said input andoutput stages, and shielding means interposed between said input andoutput circuits, said shielding means arranged to control the amount andselect the phase of electromagnetic coupling existing between said inputand output circuits.

2. In a multi-stage transformer coupled high frequency amplifier systemhaving input and output stages; the combination of, a main chassismember having a series of tube sockets and transformer holes cut thereinin two parallel lines, said tube sockets and transformer holes beingadapted to receive the corresponding components of said amplifier insuch an order as to place the input and output stages of said amplifierat adjacent ends of said parallel lines, the connections to successivestages of said amplifier being poled to produce phase opposed magneticfields associated with said input and output stages, selected lead wiresin said input and output circuits being disposed beneath the chassis andlooped in the form of single turn inductances to provide degenerativecoupling between said input and output stages, and a shieldin meansdisposed beneath said chassis member adjacent to said output stage, saidshielding means comprising a flat metallic member extending at rightangles to said chassis member.

3. In a multi-stage transformer coupled high frequency amplifier systemhaving input and output stages; the combination of a main chassis memberhaving a series of tube sockets and transformer holes cut therein in twoparallel lines, said tube sockets and transformer holes being adapted toreceive the corresponding components of said amplifier in such an orderas to place the input and output stages of said amplifier at adjacentends of said parallel lines, the connections to successive stages ofsaid amplifier being poled to produce phase opposed magnetic fieldsassociated with said input and output stages, selected lead wires insaid input and output circuits being disposed beneath the chassis andlooped in the form of single turn inductances to provide degenerativecoupling between said input and output stages, the lead wires connectingan input signal to said input transformer being disposed beneath saidchassis member, and a shielding means to limit the amount ofdegenerative coupling comprising a fiat metallic member so disposedbeneath said chassis member with its principal plane parallel to saidchassis member as to provide a space between itself and said chassismember in which said lead wire may be housed and shielded thereby.

4. In a multi-stage transformer coupled high frequency amplifier systemhaving input and output stages; the combination of, a main chassismember, a pair of grounding fixtures of the class described mounted inparallelism on the under side of said main chassis member and adapted toreceive the components of said amplifier in such an order as to placethe input and output stages of said amplifier at adjacent ends of saidfixtures, the connections to successive stages of said amplifier beingpoled to produce phase opposed magnetic fields associated with saidinput and output stages, selected lead Wires in said input and outputcircuits being disposed beneath the chassis and looped in the form ofsingle turn inductances to provide degenerative coupling between saidinput and output stages, and a shielding means disposed beneath saidchassis member adjacent to said output stage, said shielding meanscomprising a fiat metallic member extending at right angles to saidchassis member.

EMERICK TOTH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,940,769 Potter Dec. 26, 19332,066,674 Dunmore Jan. 5, 1937 2,232,064 Hannemann Feb. 18, 19412,245,379 Barton June 10, 1941

